1. Field of the Invention
The present invention relates to an optical information recording medium and dithiolate transition metal complex compounds employed in the optical information recording medium. More specifically, the present invention relates to an optical information recording medium capable of performing efficient writing operation by laser beams, in particular, by semiconductor laser beams, and dithiolate transition metal complex compounds which are effective as a near-infrared-light absorption agent and a light stabilizer for use in the optical information recording medium.
2. Discussion of Background
In the field of information recording and reproducing, an information recording and reproducing apparatus capable of recording and reproducing information by applying a laser beam to a rotating disc-type information recording medium is conventionally known. For the above-mentioned information recording and reproducing apparatus, there are proposed, for example, information recording media which comprise a substrate and a recording layer formed thereon, comprising a low-melting metal, or a mixture of a low-melting metal and a dielectric material. However, the above information recording media have the shortcomings of poor preservability, insufficient resolution, low recording density and high manufacturing cost.
An information recording medium has been proposed, which is so constructed that an organic dye-containing thin-film comprising as the main component a polymethine dye, serving as a recording layer, is overlaid on a substrate. This information recording medium has an improved writing sensitivity and reflectance. Accordingly, it has an improved C/N ratio in reading operation. The above recording medium, however, has the shortcomings of poor stability to light and heat, unsatisfactory preservability and remarkable deterioration of the recording reproduction capability during repeated use.
In order to prevent the deterioration of the recording reproduction capability in the information recording medium, it is proposed that a nickel complex compound be added to the above-mentioned polymethine dye in the composition of the recording layer. However, the recording medium which comprises a recording layer comprising the polymethine dye and the above-mentioned conventional nickel complex compound is still unsatisfactory for use in practice because of high pit-error ratio and insufficient retention of signals for the reproduction of recorded information in the course of repeated actual operation.
With the progress of the development of the apparatus such as an optical recording medium, an infrared filter, a liquid-crystal display, a laser printer and various apparatus in the field of electrophotography, as previously mentioned, a demand for a dye-containing composition with high stability to light, capable of absorbing the light in a region from an infrared region over a near infrared region with high efficiency is recently increasing.
Conventionally, as a dye for the above-mentioned dye-containing composition capable of absorbing the light in the near infrared region, squarium-type dyes, cyanine-type dyes, metal phthalocyanines, platinum bis(dithio-.alpha.-diketone) complex compounds, and nickel complex compounds are known. However, these conventional dyes have their own shortcomings. For example, the squarium-type dyes have the shortcomings that the light absorbing intensity is poor, and they cannot be worked into a film; and the cyanine-type dyes have the shortcomings that they become chemically unstable when their molecular chains are elongated to improve the near infrared light absording capability. Furthermore, the metal phthalocyanine dyes show such a poor solubility in organic solvents that they cannot be worked into a thin film by the solution coating method. In addition, an amorphous film of the metal phthalocyanine dyes is easily crystallized, so that its application to the optical information recording medium is considerably restricted. The platinum bis(dithio-.alpha.-diketone) complex compounds have the shortcomings that their film-forming properties are poor and chemically unstable.
As for the conventional nickel complex compounds, benzenethiolate-type nickel complex compounds and dithiolene-type nickel complex compounds are in particular well-known. They cannot satisfy all the requirements such as solubility in solvents, compatibility with resins, film-forming properties and chemical stability.
Among the nickel complex compounds, dithiolate nickel complex compounds have a distinct property of deactivating oxygen in a singlet state, so that some attention is paid to the dithiolate nickel complex compounds as a practical light stabilizer and antioxidant for polyolefins, and as a photo-deterioration preventing agent for the organic dyestuff. However, the solubility in solvents, compatibility with resins and film-forming properties of these dithiolate nickel complex compounds are insufficient for use in practice.